High-velocity oxy-fuel (HVOF) flame spray coating is known as a technique for forming an amorphous phase on a surface of a base material. This technique is as follows. By feeding a fuel and an oxygen gas from the body of a flame spray gun, flame (gas flame) is jetted forward at a high velocity. Particles (powder) of a flame spray material are fed to the flame with the use of a carrier gas. The particles fed to the flame are heated while being accelerated in the flame, strike a surface of a base material along with the flame, and are cooled to be solidified on the surface. Thus, an amorphous coating film is formed on the surface of the base material depending on the type of the metal, which is determined by the components of the particles, and on the cooling speed at which the particles are cooled and solidified. The following Patent Documents 1 and 2 describe high-velocity oxy-fuel flame spray coating.
In this high-velocity oxy-fuel flame spray coating, the particles dwell in the flame only for a short time, so that they are difficult to be melted completely. Moreover, the temperature of the base material rises, so that the cooling speed tends to become lower. For these reasons, materials that can be used to form amorphous coating films have been limited only to those metals whose melting points are low and whose abilities to become amorphous are great. For example, metal glasses with melting points of about 1200K or less and supercooling temperature ranges of 50K or more come under such metals.
The following Patent Document 3 describes an apparatus that makes it possible to use a variety of metals, not limited to metal glasses, to form amorphous coating films. This apparatus is illustrated in FIG. 12 in the drawings attached hereto. Flame F containing particles of a flame spray material is jetted from a flame spray gun 10′ toward a base material M, and a cooling gas G is blown around the flame F. The cooling gas G is not only blown along a nozzle 11′ of the flame spray gun 10′, but also ejected from a plurality of conduits 20′ provided around the flame F so as to come close to the flame F. In such an apparatus for flame spray coating, since the flame F is cooled before it reaches the base material M, the particles of the flame spray material can readily become amorphous. Therefore, even with the use of a metal having a high melting point and a narrow supercooling temperature range as a flame spray material, it is possible to form an amorphous coating film on the base material M.    Patent Document 1: JP 2006-159108A    Patent Document 2: JP 2006-214000A    Patent Document 3: JP 2008-43869A
In the apparatus described in Patent Document 3, there is still room for improvement in respect of the following points.
a) There is a space between each two of the conduits 20′ shown in FIG. 12, so that the flame F is partly exposed to an atmospheric air in the stage where particles of the flame spray material are melted (the stage before the particles are cooled by means of the cooling gas). The particles, therefore, tend to undergo oxidation.
b) A plurality of the conduits 20′, protruded around the path along which the flame F is jetted, inevitably make the apparatus large in size. Although on-site operation can be done with the apparatus, it is not easy to handle the apparatus.